FIG. 13 is a cross-sectional view showing a structure of a conventional developing device.
A developing device used for an image forming apparatus employing an electrophotographic system such as a copier includes a developing roller 101 for having a magnet roller 100 which transports two-element developer and developing electrostatic latent images held by a photosensitive drum, a regulating member 102 for regulating an amount of the developer transported to the electrostatic latent image with the developing roller 101, a reflux plate 103 for refluxing excess developer which is regulated by the regulating member 102 away from the regulating member 102, a hopper 104 in which the developer is contained, and a agitating roller 105 for agitating the developer in the hopper 104 (for example, see Japanese Patent Application Laid-Open No. 1-237577 (1989)).
The developing roller 101 includes the nonrotational magnet roller 100 in which magnetic poles N and magnetic poles S are alternatively arranged in the circumferential direction thereof and a nonmagnetic sleeve 106 which is rotatably fitted to the magnet roller 100.
The regulating member 102 is arranged near a developing range facing the developing roller 101, and the reflux plate 103 is provided at the opposite side of the regulating member 102 across the developing range.
According to the developing device having such structure, two-component carrier (magnetic power) and toner contained in the hopper 104 are agitated by the agitating roller 105 and toner is attached to circumference of the carrier. Further, as the sleeve 106 of the developing roller 101 rotates in opposite direction of the rotation of the photosensitive drum, developer between the developing roller 101 and the reflux plate 103 is transported toward a gap and the amount of the developer transported to the electrostatic latent image is regulated by the regulating member 102. The developer passed through the gap is transported to the electrostatic latent image. The excess developer regulated by the regulating member 102 accumulates in an interspace between the regulating member 102 and the reflux plate 103. As the accumulation amount increases, the developer flows toward upper surface of the reflux plate 103, is guided by the reflux plate 103, and refluxes toward the upper side of the agitating roller 105.
As developing devices in which two-component developer is employed, a developing device including a refluxing unit for refluxing excess developer which is regulated by a regulating member toward the upper side of an agitating roller and a reflux plate having a folded portion in one end of the refluxing unit is also well known (for example, see Japanese Patent Application Laid-Open No. 3-89273 (1991)).
However, there have been problems in the developing device disclosed in H No. 1-237577 (1989) having a structure in which the reflux plate 103 for refluxing excess developer away from the regulating member 102 is provided. The developer is transported to the gap without preliminary electrification in positive manner and the transportation amount of the developer is regulated by the regulating member 102. Accordingly, carrier of the developer transported to the gap is in state of chain composition with relatively strong force by the magnetic attraction force of the magnet roller 100. The part of the developer in chain composition state becomes excess developer by the regulation of the regulating member 102. As the excess developer increases, a ball of the developer in chain composition state may grow. The grown ball of developer drops into the downstream side of the reflux plate 103 because of the own weight, and thus, a developer circulation path toward the developer supply unit in the hopper 104 is not formed. Further, the number of turning on and off of the developer magnetic permeability sensor provided at the bottom of the hopper 104 increases so that the changes in electrification amount of the developer increases.